As shown in FIG. 1, a work vehicle 1 such as a forklift is provided with a hydro-static transmission (HST) 17. In such a work vehicle 1, a speed change is performed by adjusting each tilt angle of a swash plate 16c of an HST hydraulic pump 16 and a swash plate 18c of an HST hydraulic motor 18 both of which the HST 17 comprises to change each displacement.
The HST-mounted work vehicle 1 is provided with accelerator operating means (such as an accelerator pedal) 25, brake operating means (such as a brake pedal) 26 and a working machine operating lever 6. When the accelerator operating means 25 is depressed, an engine 8 is controlled so as to obtain an engine speed Ne corresponding to an accelerator opening. The brake operating means 26 also has an inching function. When the brake operating means 26 is depressed, the swash plate 16c of the HST hydraulic pump 16 is adjusted so that, in accordance with the increase of a stroke S of the brake operating means 26, a mechanical brake ratio BR as the brake force of a brake device 22 as a mechanical brake increases and an inching rate IR decreases.
Here, the inching rate IR means a distribution ratio of a driving force of the engine 8 to the HST hydraulic pump 16. And, the more the inching rate IR decreases, the more the distribution ratio of the driving force of the engine 8 to the HST hydraulic pump 16 decreases and the more the distribution ratio to the working machine hydraulic pump 9 increases.
FIG. 2 shows a characteristic L1 of a relationship between the brake stroke S and the mechanical brake ratio BR, and a characteristic L2 of a relationship between the brake stroke S and the inching rate IR.
FIG. 3 shows characteristics L3 and L3′ of a relationship between the engine speed Ne and an absorption torque T of the HST pump 16 and, when the inching rate IR decreases with a decrease in the displacement of the HST hydraulic pump 16, the characteristic L3 of a large absorption torque T indicated by a solid line changes accordingly to the characteristic L3′ of a small absorption torque T indicated by a chain double-dashed line.
When an operator depresses the brake operating means 26, the mechanical brake ratio BR increases in accordance with the increase of the brake stroke 5, and the brake force of the brake device 22 increases in accordance with the characteristic L1 shown in FIG. 2. And, the displacement of the HST hydraulic pump 16 decreases in accordance with the increase of the brake stroke S and the inching rate IR decreases in accordance with the characteristic L2 shown in FIG. 2. Accordingly, the absorption torque T of the HST hydraulic pump 16 decreases as shown in FIG. 3. The absorption torque T of the HST hydraulic pump 16 is decreased in order to prevent engine stall. In FIG. 2, the section in which both the mechanical brake ratio BR and the inching rate IR are larger than zero is called a “lap section”. The lap section is determined taking an operation feeling and the like of the brake operating means 26 into consideration so as to obtain optimum characteristics L1 and L2.
When the brake operating means 26 is depressed and the inching rate IR decreases, a reverse driving force (braking force) is transmitted from drive wheels 24 to the engine side to apply the engine brake, and the speed Ne of the engine 8 exceeds a prescribed maximum rotation speed NeH (overrun of the engine 8). This happens due to decrease in the displacement of the HST hydraulic pump 16.
The overrun of the engine 8 may damage the engine 8 and components (auxiliaries) around the engine 8 and thus, should be avoided.
Conventionally, the overrun of the engine 8 was prevented by advancing the action of the mechanical brake by the brake device 22 to absorb the driving force from the drive wheels 24 by the brake device 22. That is, in FIG. 2, a lap amount is formed largely by shifting the characteristic L1 to the left in the drawing to a characteristic L1″ (indicated by a broken line) to raise the mechanical brake ratio BR in the early stage of the brake stroke S.
Patent references showing a general technical level related to the present invention are as follows.
The following Patent Reference 1 discloses an invention in which, if a work vehicle such as an HST-mounted wheel shovel shows a sign of overrun, a braking action is attained by increasing the displacement of a traveling motor to generate a brake pressure in a main circuit on the outlet side of the traveling motor, subjecting the brake pressure to torque conversion by a hydraulic pump, and absorbing it by the engine.
The following Patent Reference 2 discloses an invention in which a work vehicle such as an HST-mounted wheel shovel has a brake motor with a brake device connected to the drive shaft of a hydraulic pump, and if the vehicle has signs of overrun, the overrun is prevented by increasing the displacement of a traveling motor to generate a brake pressure in a main circuit on the outlet side of the traveling motor, increasing the displacement of the brake motor to absorb its torque by the engine, and generating brake torque.    Patent Reference 1: Japanese Patent Application Laid-Open No. 2001-235032    Patent Reference 2: Japanese Patent Application Laid-Open No. 2009-24747